Design and build is a term describing a procurement route in which a single contractor is appointed to design (or complete the design) and then to construct the works. This is as opposed to a traditional contract, where the client appoints consultants to undertake the design and then a contractor is appointed to construct the works.
Design and build can be advantageous in terms of time as it allows the overlap of design and construction reducing the overall project delivery time.
The fact that there is a single point of responsibility for the client to deal with once the contract is awarded can also make things simpler. The client is also able to engage with the contractor and their design team or supply chain earlier in the design process to try and achieve a more practical or buildable solution if necessary.